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systemd for developers

systemd for developers. Alison Chaiken alison@she-devel.com http://she-devel.com Feb. 21, 2015. Text in blue is hyperlinked. On-the-fly audience exercises. Quiz : what is the most widely used Linux init system?. ?. ?. ?. ?. ?. ?. ?. ?. Topics. Motivation Design of systemd

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systemd for developers

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  1. systemd for developers Alison Chaiken alison@she-devel.com http://she-devel.com Feb. 21, 2015 Text in blue is hyperlinked. On-the-fly audience exercises.

  2. Quiz: what is the most widely used Linux init system? ? ? ? ? ? ? ? ?

  3. Topics Motivation Design of systemd Comparison with sysVinit Integration of systemd with kernel features Some tips for users and developers

  4. Linux needs to keep innovating “No one has a guaranteed position in the technology industry.” -- Bill Gates, Pirates of Silicon Valley “The only thing that can ever hurt Linux is Linux itself.” -- GKH, Linux Action Show “Success is a self-correcting phenomenom.” -- Gary Hamel Licensed under CC BY-SA 3.0 http://commons.wikimedia.org/wiki/File:Fire-lite-bg-10.jpg#mediaviewer/File:Fire-lite-bg-10.jpg

  5. Design

  6. Philosophy Extract duplicate functionality from individual daemons and move it to the systemd core or the Linux kernel. Replace /etc scripts with declarative configuration files in a standard format.

  7. One daemon to rule them all xinetd: a daemon to lazily launch internet services when activity is detected on an AF_INET socket systemd: a daemon to lazily launch any system service when activity is detected on an AF_UNIX socket (oversimplification)

  8. modular; asynchronous and concurrent; described by declarative sets of properties; bundled with analysis tools and tests; features a fully language-agnostic API. systemd is:

  9. sysVinit runlevels ≈ systemd targets Targets are synchronization points for boot. Check /lib/systemd/system/runlevel?.target symlinks: multi-user.target.wants (runlevel 3 == text session) graphical.target.wants (runlevel 5 == graphical session) Select boot-target : via /etc/systemd/system/default.target symlink; appending number ('3' or '5') or systemd.unit=<target> to kernel cmdline; Change current target with runlevel,telinit or systemctl isolate <something>.target

  10. init.d scriptsÞsystemd units Unit's action and parameters: ExecStart= Dependencies: Before=, After=, Requires=, Conflicts= and Wants=. Default dependencies: Requires= and After= on basic.target; Conflicts= and Before= on shutdown.target. Types of unit files: service, socket, device, mount, scope, slice, automount, swap, target, path, timer, snapshot

  11. Understanding dependencies Try: systemctl list-dependencies basic.target systemctl list-dependencies –after tmp.mount

  12. Understanding dependencies, p. 2 Try: systemd-analyze dot rescue.target systemd-analyze dot basic.target > basic.dot dot -Tsvg basic.dot -o basic.svg eog basic.svg (or view basic.svg with any web browser)

  13. Hierarchy of unit files forsystem and user sessions Organized into system and user units /lib/systemd/system: systemd upstream defaults for system-wide services /etc/system/system: local customizations by override and extension /lib/systemd/user/: systemd's upstream defaults for per-user services $HOME/.local/share/systemd/user/ for user-installed units 'drop-ins' are run-time extensions

  14. [system and user units: gnome-weather demo]

  15. systemd sysVinit Comparison with sysVinit

  16. SysV already has a big service manager: bash [user@localhost]$ wc -l /sbin/init 64 [user@localhost]$ wc -l /bin/bash 4154 [user@localhost]$ wc -l /lib/systemd/systemd 5944

  17. which services are started by sysVinit? Try: 'ls/etc/init.d'

  18. Which daemons started by systemd directly? Try: 'ls /lib/systemd/system/*.service' Try: 'systemctl list-sockets'

  19. Major Differences with SysVInit clean environment socket-based activation

  20. Serial Linked list Fully parallel X Upstart

  21. [Socket activation demo with cups and ncat]

  22. using the systemd journal Run “addgroup $USER systemd-journal” for access. Can be cryptographically signed. Log-reading tools are simple: Try: journalctl -xn journalctl -p err journalctl -u cron journalctl --list-boots systemctl status systemctl is-failed bluetooth systemctl --failed

  23. integration of systemd with kernel features

  24. systemd and cgroups • cgroups are a kernel-level mechanism for allocating resources like storage, memory, CPU and network • systemd slices are groups of daemons whose resources are managed jointly. • systemd scopes are similar groups of user processes. • Can set BlockIOWeight, IOSchedulingPriority, OOMScoreAdjust, CPUShares, MemoryLimit … Try: sudo systemd-cgls sudo systemd-cgtop

  25. systemd and udev • udev is a kernel facility that handles device events. • merged into the systemd project. • Rules are enabled by placement in /lib/udev/rules.d, unlike systemd unit enablement. • Rule loading is ordered by numeric filename prefix, like old sysVinit scripts.

  26. Try: ls /lib/udev/rules.d cat /lib/udev/rules.d/99-systemd.rules udev is still old-school

  27. systemd and security: granular encapsulationvia kernel's capabilities • PrivateTmp, PrivateDevices, PrivateNetwork • JoinNamespaces • ProtectSystem (/usr and /etc), ProtectHome • ReadOnlyDirectories, InaccessibleDirectories • systemd-nspawn: systemd's native containers • Easy configuration of kernel's capability properties

  28. developing systemd • git clone git://anongit.freedesktop.org/systemd/systemd • systemd-devel list: submit patches or ask questions • Impressive and featureful utility library in src/shared/ #define streq(a,b) (strcmp((a),(b)) == 0) #define strneq(a, b, n) (strncmp((a), (b), (n)) == 0) #define strcaseeq(a,b) (strcasecmp((a),(b)) == 0) #define strncaseeq(a, b, n) (strncasecmp((a), (b), (n)) == 0) • Complex but automated build system with many dependencies. • 'Plumbing' dev tools in /lib/systemd, 'porcelain' tools in /bin find /lib/systemd -executable -type f

  29. Summary Systemd has: a superior design; tight integration with the Linux kernel; a vibrant developer community. Control has migrated away from distros toward kernel and freedesktop.org. Most users will not notice. systemd exemplifies the modernization Linux needs to stay relevant and competitive.

  30. Resources Man pages are part of systemd git repo. freedesktop.org: systemd mailing list archives and wiki At Poettering's 0pointer.de blog ➟At wayback machine: “Booting up” articles Neil Brown series at LWN ➟Fedora'sSysVinit to systemd cheatsheet Steve Smethurst'sHacker Public Radio episode Josh Triplett's Debconf talk video Linux Action Show interviews with Mark Shuttleworth and Lennart Poettering

  31. Thanks Mentor Graphics for sending me to Germany to hack on systemd. Vladimir Pantelic, Tom Gundersen and Lennart Poettering for corrections of an earlier version (without implied 'ack'). Ivan Shapovalov and Mantas Mikulènas for answering questions. Bill Wardand Jym Dyer for use of their images.

  32. photo courtesy Jym Dyer

  33. Leftover Materials

  34. Greg K-H: “Tightly-coupled components”

  35. Modularity can produce complexity photo courtesy Bill Ward

  36. systemd and outside projects: CoreOS • networkd was initially contributed by CoreOS developers. • CoreOS's fleet “tool that presents your entire cluster as a single init system” is based on systemd. • Spin up new containers due to events on sockets. • CoreOS devs are outside systemd inner circle. • systemd has many patches from Arch, Intel, Debian . . .

  37. systemd in embedded systems • systemd is widely adopted in embedded systems because • proper allocation of resources is critical; • fastboot is required; • customization of boot sequence is common. • Lack of backward compatibility for older kernels (due to firmware loading) is a pain point. • Embedded use cases are not always understood by systemd devs.

  38. [runlevel demo with Fedora Qemu and Firefox] Try: 'systemctl isolate multi-user.target' [warning: KILLS X11]

  39. systemd is easy to use systemd utilities: Try: apropos systemd | grep ctl All-ASCII configuration files: no hidden “registry”. Customization is by overriding default files. Many choices are controllable via symlinks. Bash-completion by default. Backwards compatibility with SysVinit

  40. Override your defaults! • Replace a unit in /lib (upstream) by creating one of the same name in /etc (local changes). • Add services to boot by symlinking them into /etc/systemd/system/default.target.wants. • 'mask' unit with link to /dev/null. • Best practice: do not change the files in /lib/systemd. • Read in-use unit with 'systemctl cat'. photo courtesy Jym Dyer

  41. Extensions: drop-ins Try: systemd-delta Try: systemctl cat <list from 1st command>

  42. Crux of the problem:Dave Neary “There is no freedesktop.org process for proposing standards, identifying those which are proposals and those which are de facto implemented, and perhaps more importantly, there is no process for building consensus around a specification . . .” (comment regarding GNOME3)

  43. systemd is . . . the basis of Fedora, RHEL, CentOS, OpenSUSE, Ubuntu, Debian and much embedded. praised by Jordan Hubbard of FreeBSD. tightly integrated with Linux kernel cgroups. the reference implementation for udev and for kdbus userspace access.

  44. Customizing your installation Replace a unit in /lib (upstream) by creating one of the same name in /etc (local changes). Add services to boot by symlinking them into /etc/systemd/system/default.target.wants. Best practice: do not change the files in /lib/systemd

  45. Sequence of targets on a typical system >$ ls -l /lib/systemd/system/default.target /lib/systemd/system/default.target -> graphical.target >$ cat /lib/systemd/system/graphical.target After=multi-user.target >$ cat /lib/systemd/system/multi-user.target After=basic.target >$ cat /lib/systemd/system/basic.target After=sysinit.target sockets.target timers.target paths.target slices.target

  46. Example: set display manager [user@localhost ~]$ ls -l `locate display-manager.service` lrwxrwxrwx. 1 root root 35 Dec 11 2013 /etc/systemd/system/display-manager.service -> /usr/lib/systemd/system/gdm.service [user@localhost ~]$ cat /usr/lib/systemd/system/gdm.service [Unit] Description=GNOME Display Manager [ . . . ] [Install] Alias=display-manager.service or WantedBy=graphical.target

  47. sysinit, sockets and multi-userare composite targets >$ ls /lib/systemd/system/multi-user.target.wants/ dbus.service@ systemd-ask-password-wall.path@ systemd- update-utmp-runlevel.service@ getty.target@ >$ ls /lib/systemd/system/sockets.target.wants: dbus.socket@ systemd-shutdownd.socket@ systemd-initctl.socket@ systemd-udevd-control.socket@ >$ ls /lib/systemd/system/sysinit.target.wants: cryptsetup.target@ systemd-journald.service@ debian-fixup.service@ systemd-journal-flush.service@ Symlinks replace lines of conditional code in SysVinit scripts.

  48. Example: change the default target [alison@localhost ~]$ ls /etc/systemd/system/default.target /etc/systemd/system/default.target -> /lib/systemd/system/graphical.target [alison@localhost ~]$ sudo rm /etc/systemd/system/default.target [alison@localhost ~]$ sudo ln -s /lib/systemd/system/multi-user.target /etc/systemd/system/default.target [alison@localhost ~]$ ~/bin/systemd-delta [ . . . ] [REDIRECTED] /etc/systemd/system/default.target → /usr/lib/systemd/system/default.target

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